Push much of the SkColorSpace_Base interface up to SkColorSpace

Some pieces still remain, but the next step looks less mechanical,
so I wanted to land this piece independently.

Bug: skia:
Change-Id: Ie63afcfa08af2f6e4996911fa2225c43441dbfb2
Reviewed-on: https://skia-review.googlesource.com/84120
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/dm/DM.cpp b/dm/DM.cpp
index ebd2fe4..4deaa2f 100644
--- a/dm/DM.cpp
+++ b/dm/DM.cpp
@@ -919,7 +919,7 @@
 }
 
 static sk_sp<SkColorSpace> rgb_to_gbr() {
-    return as_CSB(SkColorSpace::MakeSRGB())->makeColorSpin();
+    return SkColorSpace::MakeSRGB()->makeColorSpin();
 }
 
 static Sink* create_via(const SkString& tag, Sink* wrapped) {